1. Field of the Invention
The present invention relates to a voltage controlled oscillator using a Colpitts circuit.
2. Description of the Related Art
As a voltage controlled oscillator (VCO), as illustrated in FIG. 20 for example, there is known a configuration using a Colpitts circuit which includes a resonance part 11 including a varicap diode VD in which a capacitance changes according to control voltage and an inductance element, and a feedback part 2 constituted of a transistor 21 which is an amplifying part and two capacitors C1, C2. In this example, a frequency signal resonated in the resonance part 11 is amplified by the transistor 21 and is fed back to a series resonance circuit via the feedback part 2, thereby forming an oscillation loop. In addition, symbol 31 in FIG. 20 denotes a buffer amplifier amplifying the frequency signal and outputs the amplified signal to an external part. Further, symbols 16, T3, and L denote an input terminal, an output terminal part, and an inductance element, respectively.
This VCO is disposed on a base substrate 5 formed of a ceramic such as LTCC (Low Temperature Co-fired Ceramic) mainly constituted of alumina (Al2O3) for example, as illustrated schematically in FIG. 2. Electronic parts corresponding to the above-described capacitors C, a circuit part 3, and so on are mounted on the base substrate 5, and these electronic parts are electrically connected via, for example, a conductive line 6 or the like. Incidentally, as already described, adjustment of the oscillation frequency (output frequency) is performed by changing voltage applied to the varicap diode VD and changing a capacitance value. However, as the frequency gets higher, the inductance value of the conductive line 6 between a connection point of the capacitors C1, C2 which are feedback capacitance elements and an emitter of the transistor increases. Then this inductance value operates to cancel out the amount of a capacitance value change of the varicap diode VD, and consequently an adjustable range of the oscillation frequency becomes smaller than a design value. Accordingly, when designing a VCO of 10 GHz for example, there is a problem that the adjustable range of the oscillation frequency cannot be secured widely. In Patent Documents 1 to 3, voltage controlled oscillators are described, but the above-described problem in the VCO using the Colpitts circuit is not considered.
[Patent Document 1] Japanese Patent Application Laid-open No. H10-209714
[Patent Document 2] Japanese Patent Application Laid-open No. H10-215119
[Patent Document 3] Japanese Patent Application Laid-open No. 2005-072154